When a vehicle (movable object) is parked in a garage, a parking lot, or the like, it is necessary to recognize a kind of an obstacle, such as a wheel stopper, a pole, or a wall, for appropriately park the vehicle. In particular, when a vehicle is parked by moving the vehicle to a parking space backward, recognition of a kind of an obstacle is significantly important. In such a case, when a driver forgets to confirm whether a wheel stopper is located in a parking space in advance, it is hard to park the vehicle appropriately. In addition, a wheel stopper, which is low in height from a road surface, is hard to be directly and visually confirmed. For example, JP-A-2006-343309 discloses a technology related to an obstacle detection apparatus. The obstacle detection apparatus disclosed in JP-A-2006-343309 includes elements arranged in an array form for transmitting a wave and receiving a reflective wave from an obstacle. Specifically, the obstacle detection apparatus calculates a distance from an obstacle in accordance with a time lag between a time point, at which transmission of a wave starts, and a time point, at which a reflective wave is received, and calculates a direction toward an obstacle in accordance with a phase difference in each received signal. In this manner, the obstacle detection apparatus specifies a position of an obstacle and determines a kind of an obstacle such as a wheel stopper, a pole a wall, or another object.
Further, JP-A-07-92263 discloses an obstacle detection apparatus configured to emit an ultrasonic wave in a beam form downward in order to detect a wheel stopper. Specifically, the obstacle detection apparatus determines existence of a wheel stopper by detecting change in a receiving time when receiving a reflective wave from a wheel stopper, which is higher than a road surface, in a state of receiving a reflective wave from a road surface. It is noted that the obstacle detection apparatus of JP-A-2006-343309 includes multiple elements in an array form for recognizing a kind of an object and is expensive. Further, the obstacle detection apparatus of JP-A-07-92263 detects an obstacle such as a wheel stopper with low height from a road surface. Accordingly, another device needs to be separately provided for detection of an object with high height, such as a pole, a wall other than a wheel stopper. In addition, the obstacle detection apparatus is configured to detect an obstacle located closely downward. Accordingly, recognition of an obstacle may be delayed.